Polyolefins, such as polyethylenes, having high molecular weight generally have improved mechanical properties over their lower molecular weight counterparts. However, high molecular weight polyolefins can be difficult to process and costly to produce. Polyolefins with lower molecular weights generally have improved processing properties. Polyolefins having a bimodal or broad molecular weight distribution, having a high molecular weight fraction (HMWF) and a low molecular weight fraction (LMWF), are desirable because they can combine the advantageous mechanical properties of the HMWF with the improved processing properties of the LMWF.
It is highly desirable to be able to produce multimodal and/or broad molecular weight distribution (MWD) polyolefins, such as multimodal high density polyethylene (HDPE) compositions, for applications including film, pressure pipe, corrugated pipe, and blow molding, e.g., Household Industrial Containers (HIC) and Large Part Blow Molding (LPBM). These compositions ideally should have excellent processability, as evidenced by high melt strength and extrusion high specific throughput with low head pressure, as well as good mechanical properties. Slow crack resistance (SCR), demonstrated by good performance in the Environmental Stress Crack Resistance (ESCR) and Notched Constant Ligament-Stress (NCLS) tests, is particularly important for HDPE-based pipe applications and recent industry specifications, e.g., ISO PE 100, are stringent in this respect. Strong SCR performance reduces the possibility of a pipe or blow molded article failing mechanically or structurally over the course of its lifetime. Additionally, it can enable the ability to lightweight blow molded articles (e.g., drums, containers, fuel tanks) and downgauge films, reducing material consumption and yielding significant cost savings.
Recent efforts to achieve the desired property balance in multimodal HDPE compositions have involved trying to make polymers with a Broad Orthogonal Composition Distribution (BOCD), where most or all of the comonomer is incorporated in the HMWF. BOCD is thought to enhance the formation of tie chains in the HMWF, leading to improved stiffness, toughness, and SCR. Conventional attempts to make multimodal BOCD HDPE compositions have used one of two approaches: 1) multiple reactors in series or parallel, typically with Ziegler-Natta catalyst systems, or 2) post-reactor melt blending. It is difficult and costly to obtain a completely homogenized blend with either approach, and lack of homogenization is detrimental to polymer properties. Additionally, the use of multiple reactors in series or parallel is typically not efficient or cost effective. It adds substantially to the capital cost of a commercial plant and limits the production rate relative to single reactor processes, especially single reactor gas phase processes.
Bimetallic catalysts such as those disclosed in U.S. Pat. Nos. 5,032,562; 5,525,678; and EP 0,729,387 can produce bimodal polyolefins in a single reactor. These catalysts typically include a non-metallocene catalyst component and a metallocene catalyst component which produce polyolefins having different average molecular weights. U.S. Pat. No. 5,525,678, for example, discloses a bimetallic catalyst including a titanium non-metallocene component which produces a HMWF, and a zirconium metallocene component which produces a LMWF.
Pyridyldiamido transition metal complexes have also been used to polymerize olefins, e.g., U.S. Pat. No. 7,973,116. Other background references include EP 0,676,418; EP 2,003,166; WO 98/49209; WO 97/35891; WO 2007/067259; WO 2012/158260; U.S. Pat. Nos. 5,183,867; 6,995,109; 7,199,072; 7,141,632; 7,172,987; 7,129,302; 6,103,657; 6,964,937; 6,956,094; 6,828,394; 6,900,321; 8,378,029; 7,619,047; 7,855,253; 7,595,364; 8,138,113; US 2002/0142912; US 2006/275571; US 2014/0127427; US 2016/0032027; and US 2014/0127427. Publications for additional background include Sheu, Steven, “Enhanced Bimodal PE Makes the Impossible Possible,” TAPPI, October 2006, Web; and Chen, Keran et al., “Modeling and Simulation of Borstar Polyethylene Process Based on a Rigorous PC-SAFT Equation of State Model,” Ind. Eng. Chem. Res., 2014, 53, pp. 19905-19915.
There is a need for improved multimodal and/or broad MWD HDPE compositions for applications including film, pipe, and blow molding, the compositions having one or more of BOCD, strong SCR performance, improved stiffness and toughness, and excellent processing properties. Ideally, such compositions are capable of being produced in a single reactor, such as a single gas phase reactor, to increase commercial efficiency and reduce costs.